Frequency multiplication is an important function required in many frequency generation circuits used in communications and instrumentation systems. An important performance metric of these multiplication circuits is the spectral purity of the output. Undesired harmonics are preferably suppressed.
The conventional general design approach of frequency triplers is to use a non-linear circuit that generates odd-order harmonics of the input and attenuates even-order harmonics. Filtering is then used to attenuate all the harmonics except the desired third harmonic. One common example of this approach is the use of anti-parallel diodes. Under RF drive, currents in the 2 anti-parallel diodes will cancel at even-order harmonics, while the odd-order harmonics will add. Another common approach is the use of an amplifier running in compression, which generates a square waveform that is rich in odd harmonics.
A drawback of these approaches is that the circuits associated with them both generate an output at the fundamental frequency that has significantly higher power than the desired third harmonic signal. Thus, these two common approaches are inherently inefficient at creating the desired third harmonic.
Filtering and/or feed-forward cancellation may be used to null the relatively large fundamental frequency from the output spectrum. However, both filtering and feed-forward cancellation may undesirably restrict the frequency bandwidth.
Another demonstrated approach is to first double the frequency of the input signal, then mix the fundamental with the doubled frequency to generate the third harmonic. By using a single-sideband mixer, it is possible to attenuate the undesired fundamental frequency at the mixer output. Although this approach helps mitigate the large spurious at the fundamental frequency, it requires creating the second harmonic and this spurious as well as the fourth harmonic are unavoidable at the output.